The invention is based on a priority application EP 03292040.7 which is hereby incorporated by reference.
The present invention relates to the field of telecommunications and more particularly to a method and apparatus for optical signal transmission.
Today telecommunication systems mainly rely on optical transmission in the backbone network but with signal processing and switching still being electrical. Future telecommunication networks, however, will eventually incorporate all-optical signal processing and transparent optical switching. Moreover, while today's optical transmission systems use multiplexing schemes such as time division multiplexing or wavelength multiplexing with permanent or semi-permanent bandwidth allocation, future networks are expected to employ optical packet switching and statistical multiplexing. Optical packet switching means that short optical bursts are switched on an individual basis through the network.
When an optical signal is transmitted over an optical fiber link, it is subject to signal distortion due to dispersion effects (chromatic dispersion, polarization mode dispersion) and other non-linear distortion. Optical receivers in the network are thus equipped with optical compensators and/or electrical equalizers to balance the signal distortion. Such compensators and equalizers are adjustable to allow adaptation to the actual signal distortion. A good overview over adaptive optical and electrical equalization of distorted optical signals is given in H. Bülow, “Polarization Mode Dispersion Mitigation” in J. Proakis (Ed.) “Encyclopedia of Telecommunications”, 2002, Wiley, which is incorporated by reference herein.
Today, a “blind” adaptation is used, which has a feedback loop that seeks to minimize for example the bit-error rate (BER) provided by a forward-error correction (FEC) unit or maximize the eye opening in an eye diagram. Such blind adaptation, however, is very slow compared to the length of optical bursts.
In the case of transparently switched optical networks, an optical signal might be re-routed via an alternate optical link and thus experience different dispersion or might experience varying non-linear distortion in the case of changing load of optical add/drop-multiplexers. Thus adaptation within a few milliseconds or even faster would be necessary. Furthermore, in optical packet-switched networks, different optical packets on the same fiber link may come via different optical paths from different sources in the network so that adaptation to each individual optical packet within a few nanoseconds might be requested. The fast adaptation thus required is orders of magnitudes to fast for known blind adaptation schemes.
In mobile radio networks, it has been proposed to use reference signals to allow fast time and frequency synchronization, see U.S. Pat. No. 5,936,961. In this different technical field, however, one has to deal with very different technical problems at much lower bitrates as compared to optical signal transmission, such as multipath propagation, fading, power control, and signal coding.
It is thus an object of the present invention to provide an adaptation of optical compensators and/or electrical equalizers in optical receivers of an optical transmission network.